1. Field of the Invention
The present invention relates to a process for continuously preparing methyl mercaptan by reacting a reactant mixture comprising solid, liquid and/or gaseous carbon- and/or hydrogen-containing compounds with air or oxygen, and/or water and sulfur.
2. Discussion of the Background
Methyl mercaptan (a.k.a., methanethiol) is an industrially important intermediate for the synthesis of methionine and for the preparation of dimethyl sulfoxide and dimethyl sulfone.
Various conventional processes have been described for preparing methyl mercaptan. Methyl mercaptan is predominantly prepared by a methanol based process, whereby methanol and hydrogen sulfide are reacted over a catalyst consisting of an aluminum oxide support, transition metal oxides, and basic promoters. Methyl mercaptan is usually synthesized in the gas phase at temperatures between 300° C. and 500° C. and at pressures between 1 bar and 25 bar. The gas product mixture comprises the methyl mercaptan thus formed, water, unconverted methanol and hydrogen sulfide starting materials, dimethyl sulfide and dimethyl ether byproducts, and small amounts of polysulfides, such as dimethyl disulfide. Gases which are inert in the context of the reaction, for example carbon monoxide, carbon dioxide, nitrogen and hydrogen, are also present in the gas product mixture. The methyl mercaptan thus formed is removed from the gas product mixture in a plurality of distillation and wash columns operating at temperatures between 10° C. and 140° C., as described in U.S. Pat. No. 5,866,721.
Methyl mercaptan may alternatively be prepared from carbon oxides, hydrogen, sulfur and/or hydrogen sulfide. For example, methyl mercaptan may be prepared over catalysts based on alkali metal tungstates, as described in U.S. Pat. No. 4,665,242. In addition, U.S. Pat. No. 4,410,731 describes a process for preparing methyl mercaptan from carbon oxides, hydrogen, hydrogen sulfide or sulfur, and catalysts based on alkali metal molybdenum sulfides comprising transition metal oxides as promoters and aluminum oxide as a support. Furthermore, WO 2005/040082 describes a process for preparing methyl mercaptan from carbon oxides, hydrogen, hydrogen sulfide or sulfur, and catalysts based on alkali metal molybdates comprising transition metal oxides as promoters and silicon dioxide as a support. However, these processes have lower selectivities for methyl mercaptan and lower conversions of carbon oxides, as compared to the methanol based process.
An additional alternative to the methanol based process includes preparing methyl mercaptan from hydrogen and carbon disulfide or carbonyl sulfide. However, this alternative process is characterized by comparatively low selectivities for methyl mercaptan, a multitude of byproducts that are difficult and costly to remove, and the necessity of handling large amounts of byproducts including toxic carbon disulfide or carbonyl sulfide.
A commonality amongst the above-mentioned processes is that carbon compounds, such as methanol, carbon oxides, carbon disulfide or carbonyl sulfide, are required as raw materials for the preparation of methyl mercaptan. Significant costs are associated with these raw materials and corresponding processes, particularly when the product selectivities for the formation of methyl mercaptan are relatively low. Moreover, complicated purification steps are oftentimes required, whereby a multitude of secondary components cannot be recycled into the process in a cost efficient manner. As a result, selectivity for methyl mercaptan is reduced and the industrial economic viability of the process is diminished.
Processes involving the direct conversion of mixtures obtained from other chemical processes, which comprise methane or higher hydrocarbons, water, hydrogen and optional sulfur-containing compounds, to methyl mercaptan have failed to provide industrially sufficient yields and selectivities in a cost efficient manner, but rather produce a multitude of byproducts including toxic carbon disulfide.
Accordingly, there remains a critical need for a cost efficient process for continuously preparing industrially sufficient yields and selectivities of methyl mercaptan by reacting a reactant mixture comprising solid, liquid and/or gaseous carbon- and/or hydrogen-containing compounds, which may be obtained from other chemical processes, with air or oxygen, and/or water and sulfur.
Based on the relatively low yields and selectivities associated with producing methyl mercaptan according to these conventional processes, as well as the wide spectrum of undesirable and toxic reaction intermediates and byproducts produced from these conventional processes, thereby necessitating employing extensive, time consuming and costly safety measures for protecting manufacturing personnel and the environment, other skilled artisans have failed to discover a solution to this long-felt need.